The specific characteristic lipids which by thin layer chromatography identify serotypes of the M. intracellulare/M. avium/M. scrofulaceum (AIS) complex of atypical mycobacteria have been recognized as serologically inactive apolar mycoside C-peptidoglycolipids and serologically active more glycosylated polar variants. By extending the approach to other serotypes and comparing structures of apolar and polar C-mycosides on an intra- and interspecies basis we hope to establish if, in general, species specificity is determined by carbohydrate segments which modify a common simple C-mycoside core. Further extension of the structural approach to rapidly growing atypical mycobacteria should reveal if a similar relationship applies to mycosides A,B or G or, alternatively, if there is not of necessity a structural relationship between specific lipids and antigens. Features of the C-mycosides, the presence of D-alanine and their role in protective capsule formation and hence in pathogenicity, suggest that D-cycloserine by inhibiting their synthesis may reduce pathogenicity and give rise to the accumulation of a biosynthetic precursor.